Found in a broad variety of bacteria, fungi, plants, insects,
crustaceans, and mammals, which is involved in the synthesis of
betalains and melanin.

The enzyme, which is activated upon binding molecular oxygen,
can catalyze both a monophenolase reaction cycle or a diphenolase
reaction cycle.

During the monophenolase cycle, one of the bound oxygen atoms is
transferred to a monophenol (such as L-tyrosine), generating an
O-diphenol intermediate, which is subsequently oxidized to an
o-quinone and released, along with a water molecule.

The enzyme remains in an inactive deoxy state, and is restored to the
active oxy state by the binding of a new oxygen molecule.

During the diphenolase cycle the enzyme binds an external diphenol
molecule (such as L-dopa) and oxidizes it to an O-quinone that is
released along with a water molecule, leaving the enzyme in the
intermediate met state.

The enzyme then binds a second diphenol molecule and repeats the
process, ending in a deoxy state.

The second reaction is identical to that catalyzed by the related
enzyme catechol oxidase (EC 1.10.3.1).

However, the latter can not catalyze the hydroxylation or
monooxygenation of monophenols.